High-voltage current limiting protective device

ABSTRACT

A high-voltage protective device comprising a hollow, generally cylindrical, electrically insulating member having an inner bore. A pair of terminal members is mounted on the insulating member adjacent to the opposite ends. One or more fuse links or fusible elements is disposed in the inner bore and spaced therefrom with each fuse link being connected between the associated terminal members. A finely divided or pulverulent arc quenching material substantially fills the space between each fuse link and the inner bore of the associated insulating member. A supporting means is secured to said insulating member at only one location which is axially intermediate the ends of said insulating member.

I Umted States Patent 51 3,648,211

McKeithan 45] Mar. 7, 197 2 [54] HIGH-VOLTAGE CURRENT LIMITING 2,108,993 2/1938 Ramsey ..337/202 PROTECTIVE DEVICE 1,730,716 10/1929 Austin ..337/224 X 3,345,4 10 1967 L d t l ..337 202 X [72] lnventor: Wesley L. McKeithan, Pittsburgh, Pa. 83 sonar 6 a l [73] Assignee: Westinghouse Electric Corporation, Pitt- Primary EXami'IeF-Bemafd Gilheany sburgh, Pa. Assistant ExaminerDewitt M. Morgan [22] Filed: Dec. 10 1969 Attorney-A. T. Stratton and C. L. Melale [21] Appl. No.: 883,951 [57] ABSTRACT A high-voltage protective device comprising a hollow, [52] U.S. Cl. ..337/202, 337/159, 337/246 generally cylindrical, electrically insulating member having an Illt- Cl t Roll! 8 inner bore. A pair of terminal members is mounted on the inof Search sulating member adjacent to the opposite ends one o more 337/201 204, 208, 224, 227, fuse links or fusible elements is disposed in the inner bore and 295 spaced therefrom with each fuse link being connected between the associated terminal members. A finely divided or [56] Rem-em cued pulverulent arc quenching material substantially fills the space UNITED STATES PATENTS between each fuse link and the inner bore of the associated insulatmg member. A supporting means 18 secured to said insu- Mlkulecky X lating member at only one location is axially inter- 3,345,483 l0/l967 Leonard et al... ....337/202 X mediate the ends f Said insulating membet 3,309,477 3/1967 Bronikowski ....337/202 X 2,523,139 9/1950 Owens ..337/23l X 10 Claims, 1 Drawing Figure If; 44 I I 40 40C IOB x so n2 I08 LQ z): I04

ff 2 400 d PATENTED A 7 2 INVENTOR Wesley L. Mc Kenthon WITNESSES We ATTORNEY HIGH-VOLTAGE CURRENT LIMITING PROTECTIVE DEVICE CROSS REFERENCES TO RELATED APPLICATIONS Certain subject matter disclosed in the present application is also disclosed and claimed in copending application Ser. No. 883,91 1 filed concurrently by Wesley L. McKeithan and John J. Astleford and assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION This invention relates to high voltage protective devices, and, more particularly, to high voltage power fuse structures.

In the mounting or supporting of high voltage protective devices, such as power fuses, it is common practice to support each fuse at the opposite ends or terminals of the fuse structure by means of a pair of spaced insulator supports having contact means or clips disposed thereon. The insulator supports in such an arrangement are normally mounted on a base which, in turn, must be supported on a suitable supporting structure. The electrical insulation required in such an arrangement between each terminal of the power fuse which is normally maintained at a relatively high potential and the adjacent metallic supporting members, which are normally formed from an electrically connecting material and which may be at ground or zero potential, is provided by the usual insulators supports.

Such known mounting structures for high voltage power fuses have certain disadvantages with respect to the required structural members involved in a particular application and the overall space required for the mounting of each power fuse. It is therefore desirable to provide an improved construction for such protective devices which overcomes certain of the above disadvantages.

SUMMARY OF THE INVENTION In accordance with the invention, a protective device comprises a hollow, generally cylindrical, electrically insulating member or bushing having an inner bore extending axially therethrough. A pair of terminal members is mounted adjacent to the opposite ends of the insulating member with each terminal member having secured thereto means, more specifically solderless type connectors, for connecting external electrical conductors or cables to the opposite ends of the overall protective device. One or more fusible elements or fuse links is disposed in and laterally spaced from the inner bore of the insulating member. The space between each fusible element and the inner bore of the insulating member is substantially filled with a finely divided or pulverulent or granular arc quenching material which is disposed in heat transferring relation with each fusible element and the inner bore of the insulating member. Where desired, an elongated supporting member formed from electrically insulating material, which may also be gas evolving during the operation of the protective device, may also be disposed in and spaced from the inner bore of the insulating member to extend axially between the associated terminal members and to support each fusible element. In order to support the overall protective device, only one supporting bracket or member is secured to a portion of the insulating member which is axially intermediate the ends of the insulating member, preferably at approximately the midpoint or middle portion of the insulating member. The insulating member also preferably includes a plurality of axially spaced, outwardly projecting portions or petticoats between the portion of the insulating member to which the supporting member is secured and both ends of the insulating member to increase the electrical creepage insulating path or distance between each terminal member and the associated supporting bracket of the protective device which may be formed from an electrically conducting material and for other purposes as well.

It is therefore an object of this invention to provide an improved elongated, high voltage protective device which is uniquely adapted to be supported at only one location along its overall length.

BRIEF DESCRIPTION OF THE DRAWING Other objects of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawing in which the single FIGURE is a view, partly in side elevation and partly in section, of a protective device embodying the principal features of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, there is illustrated a protective device or high voltage current limiting fuse structure 10 embodying the teachings of the invention. In general, the protective device 10 is elongated in configuration and is adapted to be supported at only a single location along its overall length.

More specifically, the protective device I0 includes a hollow, generally cylindrical, electrically insulating member or bushing 40 having an axially extending inner bore, as indicated at 42. The electrically insulating member 40 may be formed from a suitable insulating material of the ceramic type, such as porcelain, or may be cast or molded from a thermosetting resin, such as an epoxy or polyester resin, or from an elastomeric material, such as butyl rubber, in either of the latter materials with a suitable filler, such as aluminum trihydrate for antitracking characteristics.

In order to support the protective device 10, more specifically, the electrically insulating member 40 at only a single location which is axially intermediate or spaced from both ends of the insulating member 40, the insulating member 40 includes at its outer periphery a portion 40A which is located axially intermediate the ends of said insulating member and which is preferably located at approximately the midpoint of the insulating member 40 or comprises the axially middle portion of said insulating member. The middle portion 40A of the insulating member 40 is adapted to receive an associated supporting means or bracket member 92 which is secured to the insulating member 40 by suitable means, such as the bolt indicated at 93 in the drawing. The bracket or support member 92, in turn, is secured to a suitable supporting means, such as the angle member 104, by suitable means such as the bolt 106. The angle member 104 may, in turn, be secured to a supporting structure, such as the wooden crossarm 102 of a utility pole, by suitable means such as the backplate H2 and the bolts 108 which pass through aligned openings in the angle member 104 and the backplate 112. Alternatively, the bracket member 92 may be secured to a supporting steel structure in a substation installation.

The insulating member 40 also includes a plurality of axially spaced, outwardly projecting portions or petticoats 44 between the middle portion 40A and each end of the insulating member 40 to increase the electrically insulating creepage path or distance between the support or bracket member 92 which may be formed from a metallic material which is also electrically conducting, such as steel, and each end of the insulating member 40, and to increase the heat radiating capability of the insulating member 40 considered as a thermal mass in the operation of the protective device 10, as will be explained in greater detail hereinafter.

In order to facilitate the assembly of an upper terminal member Tl as shown in the drawing, the upper end of the insulating member 40 includes a recess which extends around the inner bore 42 and which includes a radial slot 408 which is adapted to receive a cross pin 64 which is secured to or formed integrally with the terminal member TI. The cross pin 64 prevents rotation of the terminal member T1 and certain associated parts which will be described hereinafter after such parts are assembled with the insulating member 40. The upper terminal member Tl may be formed from any suitable electrically conducting material, such as copper, and, as illustrated, includes an externally threaded portion which projects or extends axially from the upper end of the insulating member 40. A solderless type connector 72 may be secured to the upper terminal member Tl by screwing an internally threaded portion of the connector 72 on the axially extending, threaded portion of the upper terminal member T1, as shown in the drawing. The connector 72 may include an overhanging portion which seats against the upper end of the insulating member 40 and closes off the inner bore of the insulating member 40 at the upper end of said insulating member. As shown in the drawing, the connector 72 includes an adjustable threaded member to permit electrical connections to the protective device from an external source of electrical power or alternating current voltage as indicated by the conductor Ll. In order to seal off the upper end of insulating member 40, a gasket member 62 may be disposed between an overhanging portion of the connector 72 and the upper end of insulating member 40 and compressed when the connector 72 is tightened down on the upper terminal member T1' In order to permit the making of electrical connections to the lower end of the protective device 10 from an external electrical circuit or load as indicated by the external electrical conductor or cable L2, the lower terminal member T2 is mounted or disposed adjacent to the lower end of the electrically insulating member 40 and projects axially from the lower end of the insulating member 40 as shown in the drawing. More specifically, the terminal member T2 includes a threaded stud portion which projects axially downwardly, as viewed in the drawing, and on which a centering or spacer member 97, which may be generally triangular or circular in cross section, may be assembled and retained on the threaded portion on the terminal member T2 between a shoulder portion of the terminal member T2 and a nut (not shown) which is assembled on the other side of the centering member 97. In order to hermetically seal the lower end of the inner bore 42 of the insulating member 40 and to assist in retaining the terminal member T2 in its properly assembled position, a suitable means such as thermosetting resin of the epoxy type, as indicated at 94, may be disposed in the inner bore 42 of the insulating member 40 to surround or embed the lower terminal member T2.

In order to provide an electrically conducting path between the terminal members T1 and T2 and to protect an electrical circuit which is represented by either the electrical conductor L1 or the electrical conductor L2, a fusible element or fuse link 60 is electrically connected between the terminal members T1 and T2. The fusible element 60 may be of the flat strip or ribbon type and is preferably composed of silver. The fusible element 60 includes a plurality of axially spaced notches or restricted cross-sectional areas, as indicated at 61 in the drawing, which may be V-shaped in configuration to provide a series of arcs during the operation of the protective device 10 with the sum of the arc voltages providing a current limiting effect which is known in the fuse art. As illustrated, the fusible element 60 is helically wound on an associated electrically insulating support member 28 which extends axially between the tenninals T1 and T2. The opposite ends of the fusible element 60 may be electrically connected and secured to the associated terminal members T1 and T2 by suitable means, such as brazing. In order to aid in arc extinction during the operation of the protective device 10, the insulating support member 28 may be molded or cast from a suitable electrically insulating material which also evolves gases during a circuit interrupting operation which assist in arc extinction, such as a glass-polyester material with a suitable filler, such as aluminum trihydrate, for antitracking characteristics. The insulating support member 28 may be substantially rectangular in cross section.

In order to additionally aid an arc extinction or interruption during the operation of the protective device 10 and to provide the current limiting action which is particularly desirable in protecting certain types of electrical apparatus or equipment, the space between the inner bore 42 of the insulating member 40 and the fusible member 60 or between the inner bore 42 and the combination which includes the fusible element and the associated insulating support 28, is substantially filled with a finely divided or pulverulent or granular arc quenching material 26, such as silica sand or quartz. The presence of the arc quenching material or filler 26 as indicated in the drawing is also important in preventing the occurrence of corona due to the presence of voids or air pockets which may result when a conventional fuse having its own insulating housing is assembled inside a separate bushing as in certain known structures. It is also important to note that the electrically insulating member 40 in the construction of the protective device 10 as just described performs the function of an electrically insulating housing for the fusible element 60, the insulating support member 28 where provided and the arc quenching material 26, as well as providing the necessary electrical insulation between the upper terminal T1 and the supporting bracket 92 and between the lower terminal T2 and said supporting bracket. In other words, during the normal operation of the protective device 10, both of the terminals T1 and T2 are maintained at a relatively high potential with respect to the supporting bracket 92 or with respect to ground potential. Portions of the insulating member 40 between the supporting bracket 92 and the respective terminal members T1 and T2 as indicated at 40C and 40D, respectively, are sufficient to withstand the potential applied between the respective terminals T1 and T2 and the supporting bracket 92, which may be metallic and electrically connecting as previously mentioned. The construction of the protective device 10 therefore eliminates the need for a separate pair of support insulators which are normally disposed at the ends of a conventional high voltage power fuse of the type described.

In the operation of the protective device 10, when the current flowing from a source of electrical power which may be either the circuit represented by the electrical conductor L1 or the circuit represented by the electrical conductor L2 to the load circuit represented by the other electrical conductor at the opposite end of the protective device 10 increases to an abnormal value the fusible element 60 will fuse or melt at one or more of the plurality of constricted portions 61 to result in a plurality of arc voltages which will effectively limit the current through the protective device 10 to a value which is referred to as let-through current" and which is normally much less than the available fault or short circuit current which might otherwise result. The are quenching material to the inner bore 42 of the insulating member 40 cooperates with the fusible element 60 during an interrupting operation to form a fulgurite as is well known in the fuse art. The thermal mass of the insulating member 40 is uniquely adapted to cooperate with the fusible element 60 and the arc quenching material 26 in dissipating heat during the normal operating condition of the protective device 10 to coordinate the thermal operating characteristics of the protective device 10 with the type of electrical equipment or circuit which is being protected by the protective device 10. It is important to note that the outwardly projecting portions or petticoats 44 of the insulating member 40 function as heat radiating surfaces which assist in determining the time-current characteristics of the protective device 10 considered as a current limiting fuse structure. It is also to be noted that after the fusible element 60 melts or fuses and the abnormal or fault current is interrupted by the protective device 10, the portion of the insulating member 40 between the terminal member which remains energized after the interrupting operation is sufficient to withstand the potential between the still energized terminal member T1 or T2 and the supporting bracket 92. In addition, the insulating member 40 assists in providing the required electrical insulation between the terminals T1 and T2 after the fusible element 60 has melted or fused to interrupt the abnormal current during an interrupting operation of the protective device 10.

In the assembly of the protective device 10, the terminal members T1 and T2 are formed from a suitable electrically conducting material, such as copper, and may be molded into the opposite ends of the insulating support member 28 where such a support member is provided. The fusible element 60 may then be assembled on the support member 28 by helically winding the fusible elements 60 on the support member 28 and then securing the upper and lower ends of the fusible element 60 to the associated terminal members T1 and T2 by suitable means, such as brazing. The cross pin 64 may then be assembled on the terminal member T1 and the centering member 97 as well as the associated nut may be assembled on the terminal member T2. A subassembly which includes the fusible element 60 and a support member 28 may be then assembled into the upper end of the insulating member 40, as viewed in the drawing, with the cross pin 64 being positioned within the radial slot provided in the upper end of the insulating member 30 to prevent rotation of the subassembly after the protective device is completely assembled. After the fusible element 60 and the support member 28 are assembled in the inner bore 42 of the insulating member 40, the insulating member 40 may be sealed at the upper end by the assembly of the gasket member 62 and the connector 72 on the terminal member T1 to thereby prevent the entrance of moisture or other contaminating materials into the upper end of the inner bore 42 of the insulating member 40 during long periods of service particularly in outdoor applications. After the upper end of the insulating member 40 is sealed as just described, the insulating member 40 may be positioned with the opening of the inner bore 42 adjacent to the terminal member T2 substantially reversed from the position shown in the drawing with the opening around the terminal member T2 being then at the vertically upper end of insulating member 40 to permit the filling of the inner bore 42 of the insulating member 40 with the arc quenching material 26 which may then be compacted by any suitable means, such as vibration or other methods. After the arc quenching material 26 is in position inside the inner bore of the insulating member 40, the thermosetting resin which forms the sealing member 94 and which includes any desired or suitable filler may be poured in the insulating member 40 around the terminal member T2 to cure or polymerize or place to thereby seal the lower end of the inner bore 42 of the insulating member 40 and to secure the terminal member T2 in the position shown in the drawing. The connector 74 may be then assembled on the terminal member T2 along with the gasket member 62 to complete the assembly of the protective device 10.

The protective device embodying the teachings of this invention has several advantages. For example, since an elongated protective device as disclosed is adapted for mounting at only a single location along its length, the protective device eliminates the need for a pair of separate insulator supports which would normally be provided at the opposite ends of the protective device 10 and therefore permits a relatively more compact construction of the overall mounting arrangements. In addition, utilizing the inner bore of an electrically insulating member as disclosed results in a relatively stronger housing for the fuse parts which are disposed in the inner bore of the insulating member and increases the effective thermal mass which is available to determine the time-current characteristics of the protective device considered as a current limiting fuse. Due to the increased thermal mass provided by the insulating member 40, the time-current characteristic of the protective device 10 has a more inverse characteristic than that of a conventional current limiting fuse to permit increased or greater short time overload performance of the electrical apparatus which is protected by the protective device 10. The more inverse characteristic provided by the protective device 10 is desirable since all or most types of electrical apparatus have an inverse load-time capability and the protective device 10 allows greater utilization of this capability without sacrificing a margin of protection in the overall range of fault currents. A conventional current limiting fuse is deficient in this respect due to its time-current characteristic. Other types of fuses, such as expulsion fuses or fuses employed in fuse cutouts, which are not of the current limiting type have more inverse time current characteristics than those of conventional current limiting fuses but do not have comparable interrupting characteristics or capability. The protective device 10, therefore, combines both the inverse time current characteristic which a conventional current limiting fuse does not provide and the greater interrupting capability of such a current limiting-type fuse.

As previously noted, the outwardly projecting portions or petticoats provided on the insulating member 40 serve to increase both of the electrical insulating creepage paths or distances along the outer periphery of the insulating member 40 between the supporting bracket 92 and each of the terminal members T1 and T2. The outwardly projecting portions or petticoats also function as heat radiating surfaces which increase the ability of the insulating member 40 to dissipate the heat which results during a normal operation of the protective device 10 and to permit the thermal coordination of the protective device with the equipment or circuits being protected. The larger heat radiating surface of the insulating member 40 due to the larger outer diameter of said member and the petticoats 44 provides more effective cooling than that in a conventional current limiting fuse. The applicant's construction results in a relatively higher continuous current rating for a given size or cross section of the fusible element 60 and a relatively lower let-through current than that of a conventional current limiting fuse having the same continuous current rating. in other words, the let-through current of a current limiting device of fuse varies directly with the cross-sectional area of the fusible element or elements. Since the protective device 10 permits a smaller cross-sectional area of the fusible element 60, the let-through current is therefore smaller.

Another advantage of the disclosed construction of a protective device is that each portion of the insulating member 40 between the supporting bracket 92 and each of the terminal members T1 and T2 is sufficient to support the potential applied between both of said terminal members and the supporting bracket 92 and to support the potential between one of the terminal members T1 and T2 and the supporting bracket 92 after the protective device 10 has interrupted an abnormal or fault current. A further important advantage of the protective device disclosed is that the structural strength of the insulating member 40 uniquely lends itself to being supported at only a single location along its length. 7

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit and the scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

lclaim:

1. An elongated high voltage current limiting protective device comprising a hollow, generally cylindrical, electrically insulating member having an axially extending inner bore, first and second terminal members mounted on said insulating member at the opposite ends of insulating member and projecting axially from the ends of said insulating member, one or more fuse elements disposed in said inner bore and laterally spaced from the inner bore, each of said one or more fuse elements being electrically connected between said terminal members and including a plurality of axially spaced restricted cross-sectional areas, a pulverulent arc quenching material substantially filling the space between said one or more fuse elements and said inner bore and being in heat transferring relation with both said one or more fuse elements and the inner bore of said insulating member, means secured to each of said terminal members for connecting an electrical conductor to each of said terminal members, and only one metallic supporting bracket secured to said insulating member axially intermediate the ends of said insulating member, said insulating member including a plurality of axially spaced, outwardly projecting portions between said supporting bracket and each end of said insulating member, the thermal mass of said insulating member and said outwardly projecting portions thereof which function as heat radiating surfaces during the operation of said current limiting device cooperating with said one or more fuse elements and said are quenching material to permit a smaller cross-sectional area of each of said one or more fuse elements for a particular continuous current rating of said current limiting device.

2. The combination as claimed in claim 1 wherein said pulverulent arc quenching material comprises silica sand.

3. The combination as claimed in claim 2 wherein each of said one or more fuse elements is formed from silver.

4. The combination as claimed in claim 1 wherein said connecting means secured to each terminal member comprises a solderless-type terminal connector.

5. The combination as claimed in claim 1, wherein the outer surface of said insulating member between each end of said insulating member and said metallic supporting bracket is sufficient to withstand a predetermined potential normally applied between each of said terminal members and said supporting bracket.

6. The combination as claimed in claim 5 wherein said pulverulent arc quenching material comprises silica sand and said supporting member is disposed at approximately the middle portion of said insulating member.

7. The combination as claimed in claim 1, wherein an electrically insulating support is disposed in and spaced from the inner bore of said insulating member to extend axially between said terminal members, said one or more fuse elements being mounted on said insulating support.

8. The combination as claimed in claim 7, wherein said pulverulent arc quenching material comprises silica sand.

9. The combination as claimed in claim 7, wherein each of said one or more fuse elements is helically wound on said insulating support.

10. The combination as claimed in claim 7 wherein each of said one or more fuse elements is formed from silver. 

1. An elongated high voltage current limiting protective device comprising a hollow, generally cylindrical, electrically insulating member having an axially extending inner bore, first and second terminal members mounted on said insulating member at the opposite ends of insulating member and projecting axially from the ends of said insulating member, one or more fuse elements disposed in said inner bore and laterally spaced from the inner bore, each of said one or more fuse elements being electrically connected between said terminal members and including a plurality of axially spaced restricted crosssectional areas, a pulverulent arc quenching material substantially filling the space between said one or more fuse elements and said inner bore and being in heat transferring relation with both said one or more fuse elements and the inner bore of said insulating member, means secured to each of said terminal members for connecting an electrical conductor to each of said terminal members, and only one metallic supporting bracket secured to said insulating member axially intermediate the ends of said insulating member, said insulating member including a plurality of axially spaced, outwardly projecting portions between said supporting bracket and each end of said insulating member, the thermal mass of said insulating member and said outwardly projecting portions thereof which function as heat radiating surfaces during the operation of said current limiting device cooperating with said one or more fuse elements and said arc quenching material to permit a smaller cross-sectional area of each of said one or more fuse elements for a particular continuous current rating of said current limiting device.
 2. The combination as claimed in claim 1 wherein said pulverulent arc quenching material comprises silica sand.
 3. The combination as claimed in claim 2 wherein each of said one or more fuse elements is formed from silver.
 4. The combination as claimed in claim 1 wherein said connecting means secured to each terminal member comprises a solderless-type terminal connector.
 5. The combination as claimed in claim 1, wherein the outer surface of said insulating member between each end of said insulating member and said metallic supporting bracket is sufficient to withstand a predetermined potential normally applied between each of said terminal members and said supporting bracket.
 6. The combination as claimed in claim 5 wherein said pulverulent arc quenching material comprises silica sand and said supporting member is disposed at approximately the middle portion of said insulating member.
 7. The combination as claimed in claim 1, wherein an electrically insulating support is disposed in and spaced from the inner bore of said insulating member to extend axially between said terminal members, said one or more fuse elements being mounted on said insulating support.
 8. The combination as claimed in claim 7, wherein said pulverulent arc quenching material comprises silica sand.
 9. The combination as claimed in claim 7, wherein each of said one or more fuse elements is helically wound on said insulating support.
 10. The combination as claimed in claim 7 wherein each of said one or more fuse elements is formed from silver. 